The present inventions relate generally to radio frequency transmitters with integral control, and more particularly to saturation detection and correction in transmitters, for example TDMA transmitters in mobile radio communication devices, and methods therefor.
U.S. Pat. No. 4,458,209 entitled xe2x80x9cAdaptive Power Control Circuitxe2x80x9d, assigned commonly with the present application, discloses a dual band analog control loop having an integrator with a variable response time provided by switching first and second resistors of an RC time constant. The faster time constant applies to transient operation and the slower time constant to steady-state operation. See also, U.S. Pat. No. 5,697,074 entitled xe2x80x9cDual Rate Power Control Loop For a Transmitterxe2x80x9d.
In digital control circuits, the peak detect circuit output is filtered and converted to a digital format before being input to a processor, which provides an output control signal based on a comparison of the digitized input signal with a reference signal. Like the analog control circuits discussed above, digital control circuits control the amplifier in a manner that tends to match the digitized peak-detect circuit signal with the reference signal. See for example, U.S. Pat. No. 5,287,555 entitled xe2x80x9cPower Control Circuitry for A TDMA Radio Frequency Transmitterxe2x80x9d.
In GSM and other time division multiple access (TDMA) based cellular radio communication devices, a closed loop digital integral control system sets the RF power level and controls the turn-on on turn-off response of the transmitter, which operates in a burst mode. In these radio communication applications, it is important to maintain power control throughout transmission in order to comply with telecommunication standards specifications. For example, the transmitter must ramp-up and ramp-down in a manner prescribed by the telecommunications standards to minimize spectral splatter.
In the exemplary radio communications application, when the transmitter output power becomes saturated, the integral control loop loses lock and the control signal grows excessively resulting in an uncontrolled open loop, a phenomenon known as integral windup. Under these uncontrolled open loop conditions, when the transmitter is required to ramp-down after a delay dependent upon the degree of the preceding integral windup, the control line goes low rapidly and the control loop attempts to re-lock, resulting in severe transients and spectral splatter. Another adverse result of the uncontrolled, open loop condition is the inability of the transmitter to turn off within the time interval specified by the telecommunications standards. The integral windup phenomenon is also prevalent in other closed loop integral control systems.
The various aspects, features and advantages of the present invention will become more fully apparent to those having ordinary skill in the art upon careful consideration of the following Detailed Description of the Invention with the accompanying drawings described below.